Phase Equilibria in CO2 - Multicomponent Hydrocarbon Systems: Experimental Data and an Improved Prediction Technique

Abstract

Abstract Numerical simulation of miscible EOR processes requires calculation of the phase equilibria that exist between solvent and oil over the entire solvent/oil composition range. One calculation approach is to tabulate K-values from experimental data and allow the simulator to access the table when necessary. This approach can lead to erroneous conclusions for CO2 miscible processes if there are insufficient tabulated phase equilibrium data to cover all possible compositions, in particular those cases where three phases exist at some point in the displacement. This paper presents a different approach to the problem. If a generalized equation of state (EOS) can match experimental data, then it can be used in a reservoir simulator to calculate the phase equilibria necessary for the prediction of fluid compositions, densities, and viscosities during a displacement process. Previous work has shown how a generalized Redlich-Kwong equation can be used to calculate typical hydrocarbon phase behavior relationships that exist in condensate and black oil reservoir systems. The equation parameters have been modified further for use in hydrocarbon/CO2 calculations over a wide range of CO2 concentrations. Appropriate mixing niles for description of the phase equilibria in CO2/hydrocarbon systems have been developed. Experimental binary vapor/liquid equilibrium (VLE) data have been used to evaluate the constants in the EOS for pure CO2 as well as interaction terms used within the mixing rules. Predictions of phase equilibria then have been made and compared with experimental data for a synthetic multicomponent hydrocarbon/CO2 system and a crude oil/CO2 system. Introduction Current interest in miscible EOR methods has led to the use of compositional reservoir simulators to understand and evaluate performance. An essential pan of such a simulation is a means of predicting the complex phase equilibria possible during EOR processes. However, because of computational time constraints, the numerical complexity of such a technique must be limited. In recent years, a number of relatively simple EOS have been developed and applied to hydrocarbon phase equilibria calculations. One such development is the generalized Redlich-Kwong EOS by Yarborough. This EOS has been applied to many reservoir fluid property calculations with excellent results because of its simplicity. For the simulation of CO2 miscible EOR processes, an EOS must be capable of predicting phase equilibria over a wide range of CO2 compositions. CO2/hydrocarbon mixtures can exhibit complex phase equilibria -e.g., liquid/liquid immiscibility, liquid/liquid/vapor equilibria, and asphaltene dropout. While it is highly unlikely that any simple EOS can provide accurate phase equilibria predictions for all these situations, the generalized Redlich-Kwong EOS referenced previously has been adapted to provide adequate phase equilibria predictions for CO2/hydrocarbon systems over a wide range of conditions. This adaptation involves the use of special parameters to describe pure CO2 above its critical temperature and of modified mixing rules to describe CO2/hydrocarbon mixture behavior. General functions are developed for the parameters involved to permit interpolation (or extrapolation) to other systems and conditions. Literature binary CO2/hydrocarbon VLE data were used to establish these parameters. The generalized EOS was tested through comparisons of predicted phase equilibria with experimental data for a number of binary and multicomponent CO2/hydrocarbon systems. The multicomponent systems include CO2 /synthetic oil and CO2/reservoir oil data presented in this paper. SPEJ P. 308^